Assisted ventilation with inspiratory positive pressure and with positive end-expiratory pressure (PEEP) is often used to reverse severe hypoxemia in acute respiratory failure. PEEP also may reduce cardiac output sometimes offsetting any benefit from improved arterial oxygen content. It has been assumed that PEEP reduces cardiac output because of reduced net right atrial filling pressure. However, recent studies in dogs and in humans show that the fall in cardiac output is associated with an increased net right filling pressure suggesting that myocardial function is depressed. It is our purpose to determine in anesthetized dogs what effect PEEP has on ventricular volume and contractility and to determine in what way regulation of ventricular function is altered by PEEP and by lung distension. Biventricular radioopaque markers and pressure catheters will be used to measure simultaneous ventricular volumes and net filling pressures. With this technique ventricular axial dimensions and volumes will be measured throughout the cardiac cycle and compared with net filling pressures. Coronary blood flow and metabolism will be studied by the radioactive microsphere technique and by a-v O2 and lactate differences. In order to separate reflex effects of lung distension from mechanical effects on pulmonary blood flow we also plan to study the response to distension of one lung in open chest dogs after all blood flow has been diverted through the other lung. Alterations in neural regulation of ventricular function will be investigated by blocking vagal afferents from the lung and by blocking both sympathetic and parasympathetic efferents.